This patent relates to the use of dichalcogenide compounds in silver halide photographic emulsions and coatings.
Problems with fogging having plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that not associated with the action of the image-forming exposure, and is usually expressed as "Dmin", the density obtained in the unexposed portions of the emulsion. A density, as normally measured, includes both that produced by fog and that produced by exposure to light. It is known in the art that the appearance of photographic fog related to reduction of silver ion can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, (spectral) chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
Several methods have been employed to minimize this appearance of fog. Mercury containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to combat fog. Thiosulfonate and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed. Additionally aromatic, heterocyclic, and acyclic disulfides which do not have labile sulfur or sulfide, such as those described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 3,043,696; 3,057,725; 3,062,654; 3,128,186; and 3,563,754, have been used, primarily as emulsion melt additives.
For the production of photographic photosensitive materials it is well known that many organic additives, especially aromatic dichalcogenides, are substantially insoluble in water. For that reason, the method usually employed for adding such additives to a silver halide photographic emulsion includes first dissolving the organic compound (hereinafter called solute) in an organic solvent freely miscible with water, for example, acetone, methanol, ethanol, propanol, or methyl cellosolve, and adding the solution to an emulsion.
However, these methods have many drawbacks. The use of an organic solvent freely miscible with water can reduce the surface activity of a co-present coating aid, coagulate a co-present binder, or solidify a co-present coupler, thereby markedly hindering high-speed coating. Additionally, because the dichalcogenide solute is substantially insoluble in water, rapid crystallization and/or flocculation of the solute can occur upon addition of the organic solution to the substantially aqueous emulsion melt resulting in solid defects in the photosensitive coatings. Lastly, organic solvents are dangerous to work with because of their volatility, and they have a negative impact on the Earth's ecology.
Aqueous solid particle dispersions of organic additives avoid these drawbacks and have been used in the industry. U.S. Pat. No. 4,006,025 (Swank) describes a dispersion process for sensitizing dyes employing elevated temperature (40.degree.-50.degree. C.) milling of an aqueous dye slurry containing surfactant. British Patent No. 1,570,362 (Langer et al) describes a dispersion process for photographic additives employing milling of an aqueous slurry of the additive in the presence of a surface active agent whose surface tension at 1 g/l is not less than 38 dyne/cm. These patents do not describe the use of these techniques with dichalcogenide compounds.
U.S. Pat. No. 3,397,986 (Herz and Millikan) describes the stabilization of photographic emulsions with bis(p-acylamidophenyl) disulfides. It teaches the introduction of these additives into a photographic emulsion via solutions of the additive in water miscible solvents such as ethanol or acetone or via dispersions commonly employed for photographic couplers. The latter method is taken to mean the process wherein the coupler is dissolved in a water-immiscible solvent; this oil phase is added to an aqueous phase of gelatin, surfactant and water; and the mixture is emulsified using a colloid mill or homogenizer.
There is a continuing need for more effective means of controlling fog in photographic elements. There is also a need for methods of preparing photographic elements which do not require the use of organic solvents.
According to this invention it has been found that if certain dichalcogenide compounds are introduced into a silver halide emulsion or photographic material as solid particle aqueous dispersions, their antifogging effect is significantly larger than that provided by water-miscible, organic solvent solutions or conventional coupler dispersions of the same dichalcogenides. In addition, the antifogging effectiveness of the dichalcogenides may be controlled by the size of the dichalcogenide particle in the solid particle aqueous dispersion. Further this method has a high degree of reproducibility compared to that achieved with water-miscible, organic solvent solutions.